1. Field of the Invention
The present invention relates to an input level supervisory system in an automatic gain controller, and more particularly to a system for supervising the level of input applied to a level regulator of carrier frequency terminal equipment, which has highly accurate performance and can be manufactured at low costs.
2. Description of the Prior Art
The level regulator used in carrier frequency terminal equipment of a frequency division multiplex communication system employs an electric current of a frequency outside the carrier frequency band as a supervisory current (pilot signal). Said electric current is transmitted with a constant amplitude and variations in the level of said electric current are detected to obtain equalization of characteristics deviation, change in the temperature, etc., occurring in the transmission line, terminal stations, etc. The level regulator changes the value of a variable element thereof, such as a thermistor or a field effect transistor (FET), in accordance with the level of the electric current thus detected, so as to compress the variations in the level of the electric current into smaller values, thus regulating the level of the input.
In addition to said function of automatic level control, the level regulator is further provided with a function of supervising the level of the pilot signal. The pilot signal level being supervised is utilized for exhibiting an alarm function of transmitting a signal for lighting an alarm lamp and ringing an alarm bell when the pilot signal level exceeds a range that can be regarded as normal, and; for a pilot signal level indicating function of constantly indicating and recording the pilot signal level through a meter or a recorder for improved maintenance.
Generally, supervision of the pilot signal level includes supervision of the level of input to the level regulator and supervision of the level of output therefrom. Of these types of supervision, the output level supervision has conventionally been employed. The two types of supervision are different in the supervising accuracy of the input level. It goes without saying that it is more desirable for higher accuracy to supervise the input level than to supervise the output level which has the same relation to the input level in a compressed state. Further, the International Telegraph and Telephone Consultative Committee recommends the input level supervision as a preferable one. In a conventional method of this art, the input level supervision uses an electric voltage which varies approximately in accordance with a decibel change in the level of input to the level regulator, which voltage is available from the control voltage for driving the variable element of the level regulator or a like voltage, thus supervising a pseudo input level. To be concrete, if there is a change in the input level, the change is detected so that the gain is controlled by the thermistor or the FET of the level regulator. Since the output voltage from the control circuit for driving the thermistor or the FET is somewhat mutually related to the input level, said output voltage, i.e., control voltage, is directly used to supervise a pseudo input level.
Further, an improved method of the above-mentioned conventional supervising method has been proposed by German Patent P 22 35 230.3, owned by Siemens Aktiengesellischaft. According to this method, said control voltage is input to a non-linear element of a correcting circuit to produce an electric voltage more proportional to the input level, to be used as a supervisory output for a pseudo input level. However, this method is not sufficiently high in accuracy. This is because, the resulting amount of change may be too large (an elongation of the input level) or too small (a contraction of the input level) with respect to the change in the input level to perform accurate supervision of the input level.
Further, there is another conventional method, according to which the input to the level regulator is branched. A particular narrow-band pilot signal filter is provided for extracting the pilot signal, and the pilot signal thus extracted is amplified and detected for performing supervision of the input level. To be concrete, for the level regulating operation, the output is fed back to produce a predetermined electric voltage through a control circuit, and said voltage is used to drive the variable element of the level regulator to control the gain. While, for the level supervising operation, a supervisory circuit is provided separately from said gain control loop, in which an input consisting of a voice signal and a pilot signal is applied to a pilot signal filter to extract only the pilot signal, which is then amplified and rectified to be obtained as a level supervisory output. However, since this method requires a particular narrow-band crystal filter with sharp attenuation characteristics and a particular rectifier circuit, it is disadvantageous with respect to manufacturing costs and simplicity in construction.